thread the same end through your rear sling mount
This is the complicated one! look at the OTHER end of the sling. see the plastic bit? One side has a short grab handle type attachment, the other is the main sling. between the 2 is a ridge of plastic. thread the end from the rear sling attachment through between this 'rib' and the main part of the sling.
Thread the sling end back through on the OTHER side of the rib.